The invention relates to a method and system for controlling the rewind temperature of strip metal, e.g. aluminum, in a continuous heat treatment line.
In the heat treatment of aluminum strip it is important to control the final temperature of the metal as it is rewound into coil form. This is because even at room temperature, the solutionized and quenched metal will undergo a process of microstructural transformation known as xe2x80x9cagingxe2x80x9d. Once rewound, due to the large mass of the coil and relatively small exposed surface area, the metal cools to room temperature over a period of many hours during which time the aging process continues. The aging will proceed to a greater or lesser extent depending on the initial temperature of the coil and the coil cooling rate. A certain amount of controlled aging is sometimes desirable and for that reason coils may be wound at a controlled temperature above room temperature to take advantage of this phenomenon.
Currently there is no active control of strip temperature after the furnace and quench section of a continuous heat treatment line. The rewind temperature control is dependent upon establishing a heat treatment and quench practice for each product giving a strip temperature upstream from the accumulator to achieve approximately the desired temperature at the rewind. This method is unreliable because conditions in the line affecting heat transfer, such as ambient air temperature, air circulation, metal width and roll temperatures can vary considerably resulting in rewind temperatures too high or too low. The only way to compensate for this has been to alter conditions upstream in the furnace or quench. This has the major disadvantages of providing very little ability to change conditions without potentially affecting the heat treatment of the metal, limited ability to predict or control the effect of the changes and slowness of response when furnace or cooler temperatures must be changed.
Johnson et al. U.S. Pat. No. 6,263,714, issued Jul. 24, 2001 describes a control system for a rolling mill in which metal strip is processed between an unwind reel and a rewind reel. It includes a programmed controller for controlling the system.
In Sellitto et al. U.S. Pat. No. 4,913,748, issued Apr. 3, 1990 an apparatus is described for the continuous annealing of strip metal. The rate at which the metal strip is fed through the system is regulated by a feed controller which includes a catenary loop of the metal strip. The size of the loop may be controlled such that a furnace may be operated concurrently to anneal the strip and function as an accumulator.
It is an object of the present invention to provide rapid and automated control of rewind temperature in a heat treatment line without affecting heat treatment or quench conditions upstream.
The present invention in one aspect relates to a method of controlling the recoil temperature of metal strip in a continuous heat treatment line. Hot, heat-treated metal strip is continuously passed through an accumulator system where it passes around at least two accumulator rolls in a spaced relationship. While travelling between the accumulator rolls, the metal strip is exposed to ambient cooling air. In order to control the amount of cooling, the length of metal strip exposed to the ambient cooling air is controlled. This exposed strip length is in turn controlled by varying the distance between the spaced accumulator rolls around which the strip travels. Thus for a greater amount of cooling, the spaced accumulator rolls are positioned further apart and for a lower degree of cooling the rolls are brought closer together.
Typically the invention uses an accumulator tower consisting of an upper and lower bank of rolls which banks of rolls can be brought together or moved apart to control the recoil temperature. The moving aluminum strip is passed alternately between the upper rolls and lower rolls to form a number of strands before passing out of the accumulator to the rewind reel or coiler.
The distance between the upper and lower roll sets and thus the length of metal strip subjected to ambient cooling air may conveniently be controlled by a programmed controller in response to a measured temperature of the metal strip at the rewind reel used as a feed back signal to the controller. As there may be some lag time between a change in conditions upstream, e.g. in the temperature of metal entering the accumulator, and the detection of the change of temperature at the rewind, it is preferable to also incorporate a feed forward signal to the controller. This may be accomplished by obtaining a strip temperature signal at some point prior to the accumulator and then by means of a mathematical model of the process calculate the required adjustment in strip length (accumulator spacing) to achieve the correct temperature at the rewind reel under the new conditions. The mathematical model may incorporate inputs of heat transfer coefficients, pressures and temperatures of the heat transfer media in each section of the line, as well as strip speed, thickness and width. Then using an appropriate algorithm in a programmable logic controller (PLC) a calculation is made as to the required accumulator spacing. The reference temperature for the feed forward signal may be the entry temperature to the line or a temperature taken at any other point in the line provided that a reasonably accurate model can be constructed to predict strip temperature from that point forward to the rewind reel.